In general, mechanical face seals are used in a rotating shaft/stationary housing milieu to control fluid leakage between two different pressure regions through which the shaft passes. Such seal arrangements are particularly useful with gas compressors in processing industries or pipe lines, and with centrifugal type pumps which find use in any number of areas such as, for example, petroleum and petrochemical applications, pipeline pumps, and feedwater pumps for boilers.
In U.S. Pat. No. 3,921,986 of Geary et al. there is shown one type of mechanical face seal in which a floating carbon ring is situated between an annular collar on the rotating shaft and a non-rotating sleeve mounted in the housing. The carbon ring is biased into contact with a contact ring carried by an annular collar, and the face-to-face contact provides the desired sealing. In U.S. Pat. No. 4,099,728 of Wiese there is shown a mechanical face seal in which face-to-face contact is between a relatively hard rotating sealing ring and a relatively soft stationary sealing ring having a back-up ring of hard material which minimizes distortion of the soft ring under operating conditions. The seal is biased to maintain constant face-to-face contact as the soft material wears down. In U.S. Pat. No. 4,552,368 of Wallace there is shown a mechanical face seal wherein the sealing faces of the stationary member and the rotary member are urged into contact with each other by the pressure of leaking gas. Thus, where there is no leak, the faces are not in contact and there is little wear. A feature of the Wallace seal is a mechanism for moving the faces into contact before any leak between them can occur. In each of the foregoing arrangements, the problems of pressure and thermal distortions prevent the seals from maintaining full parallelism of the contacting seal faces, and hence, from maintaining sealing integrity. In addition, such face-to-face seals have a high rate of wear and require, in most cases, disassembly of the seal and replacement of the face rings after wear has destroyed the integrity of the seal. In U.S. Pat. No. 4,792,146 of Lebeck et al. there is shown a radially compliant seal having a zero net thermal taper which insures full face-to-face contact between the sealing elements and hence uniform wear without loss of sealing integrity. However, wear does occur as a result of the sliding contact of the sealing surfaces.
An answer to the wear problem is found in the use of non-contacting seal faces, primarily for either hydrodynamic (dependent on rotation) or hydrostatic (non-dependent on rotation) seals. In U.S. Pat. No. 4,212,475 of Sedy there is shown such a non-contacting hydrodynamic face seal wherein one of the seal faces has spiral grooves cut therein to assure hydrodynamic generation of elevated pressures between the radially extending faces. The flat-faced hydrostatic mechanical seal, which, by definition, is not dependent on the shaft rotating, with controlled leakage wherein the faces are separated with a film of fluid (gas or liquid), is strongly dependent on the coning of the seal faces due to fluid pressure and thermal effects. Thus, the film stiffness is positive only if the coning is positive, where the seal gap is convergent in the direction of leakage flow, and decreases very steeply if the coning is reduced. If there is zero coning, the stiffness is zero, and if there is negative coning, it is negative, and the seal is unstable. Thus, a hydrostatic seal must be designed to operate with positive coning. The coning, which may be defined as the difference between the film thickness at the outside diameter (OD) of the seal and at the inside diameter (ID) thereof must be small to obtain a thin enough film for acceptable controlled leakage. This desired difference in thickness is generally on the order of one to two microns (1-2 .mu.). Inasmuch as mechanical and thermal deformations are of the same order, it has heretofore been difficult to engineer a seal having just the right amount of positive coning to yield a desired stiffness. Furthermore, if operating conditions change, the coning can change by a significant amount, as will the stiffness. Thus, conventional hydrostatic fluid seals in operation are at risk of operating with low or negative stiffness which can result in instability and fluid film collapse with consequent failure of the seal. The problem of coning is addressed to some extent in the aforementioned prior art patents, but primarily in the context of undesirable and irregular wear of the seal faces.